Optical disk apparatus having compensation for objective lens dislocation

ABSTRACT

The present invention provides an optical disc apparatus which solves a problem that the tracking control takes off when an objective lens is displaced in the tracking direction due to its weight. In this optical disc apparatus, a spot position detector  108  generates a spot position signal which indicates a position of a light spot on a light receiving element  105 , and supplies the signal to a traverse loop filter  111  and a spot position loop filter  109 . A controller  115  supplies an output from the spot position loop filter  109  to a tracking actuator  104 , before supplying an output from the traverse loop filter  111  to a traverse motor  113 , previously moves the light spot near the center of the light receiving element  105 , and starts the traverse control in a state where a value of the spot position signal at starting of the traverse control is reduced.

FIELD OF THE INVENTION

[0001] The present invention relates to an optical disc apparatus whichapplies a light spot to an optical disc, thereby to record or reproduceinformation.

BACKGROUND OF THE INVENTION

[0002] In recent years, optical disc apparatuses which applies a lightspot to an optical disc thereby to record or reproduce information, suchas a CD (Compact Disc) or MD (Mini Disc) have been developed. Theoptical disc has tracks on/from which the information isrecorded/reproduced and the optical disc apparatus records/reproducesthe information by making the light spot follow the tracks. The tracksare arranged concentrically or helically at intervals of severalmicrometers (1.6 μm in the case of CD or MD) in its radial direction.The optical disc apparatus has light spot moving means for moving thelight spot in a radial direction of the optical disc at high speed andwith high precision, to follow these microscopic tracks. As examples ofthe light spot moving means, a tracking actuator for radially moving anobjective lens which focuses a light spot, and a galvanomirror forchanging an angle of an incident light to the objective lens are cited.However, only with such light spot moving means, a movement range of thelight spot is limited by the size of the tracking actuator or theobjective lens or the like and accordingly the movement range is small.Therefore, the optical disc apparatus usually includes traverse movingmeans for radially moving an optical head itself which internallycontains the objective lens. Generally, control for making the lightspot follow the tracks by using the light spot moving means is called“tracking control” and control for making the optical head follow themovement of the light spot by using the traverse moving means is called“traverse control”.

[0003] In addition, in the conventional traverse control, a differencebetween the objective lens position and the center of the optical headis generated as an error signal, thereby to perform the control by usingthis error signal. A method in which a low-band component of a trackingdriving signal supplied to the tracking actuator is used as the errorsignal is widely adopted. The low-band component of the tracking drivingsignal shows displacement of the objective lens by the tracking control,on the basis of a position where weight and gravity of the objectivelens is balanced. When the moving direction of the tracking actuator ishorizontal, it is a signal corresponding to a relative position of theobjective lens and the center of the optical head, because thedisplacement of the objective lens by the gravity from the operationcenter of the tracking actuator in contrast to the moving direction ofthe tracking actuator is approximately “0” in this case. However, whenthe position of the apparatus is set up so as to have a vertical movingdirection of the tracking actuator, i.e., when the optical discapparatus is positioned “longitudinally (vertically)”, the objectivelens is displaced downwardly due to its weight, which is referred to as“self-weight dislocation”. The position of the objective lens displaceddue to the self-weight dislocation is the position where the weight andgravity of the objective lens is balanced, and the traverse control isexecuted with this position as a center. Therefore, in the conventionaloptical disc apparatus, when the self-weight dislocation occurs, themovable range of the tracking actuator is narrowed accordingly, wherebythe follow-up characteristics of the tracking control are deteriorated.

[0004] Japanese Published Patent Application No. Hei. 9-223320 disclosesan optical disc apparatus which solves such problems. The optical discapparatus disclosed therein comprises spot position signal generationmeans for generating a spot position signal which indicates relativedisplacement of a light receiving element in the optical head and alight spot, and has a structure of using the spot position signal as anerror signal of the traverse control. According to this structure, thetraverse control of this optical disc apparatus has a point where thespot position signal is zero as a control target and moves the opticalhead such that the light spot is positioned at the center of the lightreceiving element. In constituting the optical head, the center of thelight receiving element and the operation center of the trackingactuator are previously arranged so as to coincide with each other.Therefore, the objective lens is always moved around the operationcenter of the tracking actuator, thereby avoiding the deterioration ofthe follow-up characteristics of the tracking control.

[0005] However, so constructed optical disc apparatus has a problem instability of the operation at the starting of the operation of thetraverse control means. FIG. 14 is a waveform chart showing waveforms ofa spot position signal and a traverse driving signal in the conventionaloptical disc apparatus when the self-weight dislocation occurs. In FIG.14, at an operation start time 1301 of the traverse control means, theobjective lens is displaced due to the self-weight dislocation and thespot position signal has a large value A. When the spot position signalis input to the traverse control means in such a state to start thetraverse control, a driving signal 1302 applied to a traverse motor hasa high amplitude and becomes oscillatory. In the worst case, when theoptical head is moved by the traverse motor, the tracking control istaken off. This is because the frequency band of the traverse control isusually limited to several Hertz or less so as not to follow theeccentricity, the traverse motor has large inertia and it is difficultto be moved or stopped, and the like.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide an opticaldisc apparatus which allows operation of traverse control means by aspot position signal, even when the self-weight dislocation of anobjective lens occurs due to variation in the position of the opticaldisc apparatus.

[0007] Other objects and advantages of the present invention will becomeapparent from the detailed description and specific embodimentsdescribed are provided only for illustration since various additions andmodifications within the spirit and scope of the invention will beapparent to those of skill in the art from the detailed description.

[0008] According to a 1st aspect of the present invention, an opticaldisc apparatus which applies a light spot to an optical disc, thereby torecord or reproduce information on or from the optical disc, comprises:first moving means for moving the light spot applied to the opticaldisc, in a radial direction of the optical disc; an optical head havingconverging means for converging the light spot on the optical disc; spotposition detection means for generating a spot position signal whichindicates a positional difference in a radial direction of the opticaldisc, between a center of the optical head and the light spot on theoptical head; first control means for subjecting the spot positionsignal to a first processing by a spot position loop filter, andoutputting the spot position signal to the first moving means; secondmoving means for moving the optical head in a radial direction of theoptical disc; second control means for subjecting the spot positionsignal to a second processing by a traverse loop filter, and outputtingthe spot position signal to the second moving means; and systemoperation control means for operating the first control means, andthereafter operating the second control means. Therefore, even when theself-weight dislocation occurs in the objective lens, the traversecontrol using the spot position signal can be performed with stability,thereby realizing an optical disc apparatus which is considerablyeffective in practical use, particularly as a portable optical discapparatus.

[0009] According to a 2nd aspect of the present invention, an opticaldisc apparatus which applies a light spot to an optical disc, thereby torecord or reproduce information on or from the optical disc, comprises:first moving means for moving the light spot applied to the opticaldisc, in a radial direction of the optical disc; an optical head havingconverging means for converging the light spot on the optical disc; spotposition detection means for generating a spot position signal whichindicates a positional difference in a radial direction of the opticaldisc, between a center of the optical head and the light spot on theoptical head; first control means for subjecting the spot positionsignal to a first processing by a spot position loop filter, andoutputting the spot position signal to the first moving means; secondmoving means for moving the optical head in a radial direction of theoptical disc; second control means for subjecting the spot positionsignal to a second processing by a traverse loop filter, and outputtingthe spot position signal to the second moving means; spot positionsignal monitoring means for receiving the spot position signal as aninput, and outputting a first signal which indicates that the spotposition signal comes to a value smaller than a prescribed value; andsystem operation control means for operating the first control meanswhen the first signal is input, and operating the second control meansafter or simultaneously with the operation of the first control means.Therefore, the execution time for the spot position control precedent tothe traverse control can be optimized and minimized, thereby realizingan optical disc apparatus which is considerably effective in practicaluse, particularly as a portable optical disc apparatus.

[0010] According to a 3rd aspect of the present invention, an opticaldisc apparatus which applies a light spot to an optical disc, thereby torecord or reproduce information on or from the optical disc, comprises:first moving means for moving the light spot applied to the opticaldisc, in a radial direction of the optical disc; an optical head havingconverging means for converging the light spot on the optical disc; spotposition detection means for generating a spot position signal whichindicates a positional difference in a radial direction of the opticaldisc, between a center of the optical head and the light spot on theoptical head; correction signal generation means for receiving the spotposition signal as an input, and generating a correction signal forcorrecting the spot position signal; subtracting means for subtractingthe correction signal from the spot position signal; second moving meansfor moving the optical head in a radial direction of the optical disc;and second control means for subjecting an output from the subtractingmeans to a processing by a traverse loop filter, and outputting theoutput to the second moving means. Therefore, the traverse control canbe stabilized with a simple structure without using the spot positioncontrol, thereby realizing an optical disc apparatus which isconsiderably effective in practical use, particularly as a portableoptical disc apparatus.

[0011] According to a 4th aspect of the present invention, an opticaldisc apparatus which applies a light spot to an optical disc, thereby torecord and reproduce information on or from the optical disc, comprises:first moving means for moving the light spot applied to the opticaldisc, in a radial direction of the optical disc; an optical head havingconverging means for converging the light spot on the optical disc; spotposition detection means for generating a spot position signal whichindicates a positional difference in a radial direction of the opticaldisc, between a center of the optical head and the light spot on theoptical head; second moving means for moving the optical head in aradial direction of the optical disc; second control means forsubjecting the spot position signal to a processing by a traverse loopfilter, and outputting the spot position signal to the second movingmeans; and a coefficient multiplier for reducing a coefficient for thecontrol by the second control means to a value smaller than that in anormal operation time, at starting of the operation of the secondcontrol means. Therefore, the traverse control can be stabilized with asimple structure without using the spot position control, therebyrealizing an optical disc apparatus which is considerably effective inpractical use, particularly as a portable optical disc apparatus.

[0012] According to a 5th aspect of the present invention, in theoptical disc apparatus of the 1st or 2nd aspect, the first processingsubjected by the first control means is a phase-lag compensation.According to the 5th aspect, the first processing to be performed to thespot position signal comprises only the phase-lag processing, i.e.,low-band compensation processing, whereby the structure of the spotposition filter can be simplified. Therefore, even when the self-weightdislocation occurs in the objective lens, the traverse control using thespot position signal can be performed with stability, or the executiontime of the spot position control precedent to the traverse control canbe optimized and minimized, thereby realizing an optical disc apparatuswhich is considerably effective in practical use, particularly as aportable optical disc apparatus.

[0013] According to a 6th aspect of the present invention, in theoptical disc apparatus of the 5th aspect, the first processing subjectedby the first control means includes compensation for reducing anopen-loop gain at a primary resonance frequency of the first movingmeans, in addition to the phase-lag compensation. Therefore, even whenthe open-loop gain at the primary resonance frequency exceeds 0 dB, thephase margin can be secured and the spot position control is stabilized.

[0014] According to a 7th aspect of the present invention, in theoptical disc apparatus of the 1st or 2nd aspect, the first processingsubjected by the first control means is a phase-lead compensation and aphase-lag compensation, and the phase-lead compensation is started froma frequency lower than a primary resonance frequency of the first movingmeans. According to the 7th aspect, both of the phase-lead compensationand the phase-lag compensation, i.e., phase compensation and low-bandcompensation are performed as the first processing to be performed tothe spot position signal. Accordingly, the oscillations can besuppressed in a low frequency band in the servo of the spot positioncontrol, according to the spot position signal having a sensitivitywhich cannot be increased. Therefore, even when the self-weightdislocation occurs in the objective lens, the traverse control using thespot position signal can be performed with stability, or the executiontime for the spot position control precedent to the traverse control canbe optimized and minimized, thereby realizing an optical disc apparatuswhich is considerably effective in practical use, particularly as aportable optical disc apparatus.

[0015] According to a 8th aspect of the present invention, an opticaldisc apparatus which applies a light spot to an optical disc, thereby torecord or reproduce information on or from the optical disc, comprises:first moving means for moving the light spot applied to the opticaldisc, in a radial direction of the optical disc; an optical head havingconverging means for converging the light spot on the optical disc; spotposition detection means for generating a spot position signal whichindicates a positional difference in a radial direction of the opticaldisc, between a center of the optical head and the light spot on theoptical head; tracking error detection means for generating a trackingerror signal which indicates a positional dislocation between the lightspot and a track on the optical disc; first control means for subjectingthe spot position signal or the tracking error signal to a firstprocessing by a phase compensation loop filter, and outputting thesignal to the first moving means; second moving means for moving theoptical head in a radial direction of the optical disc; second controlmeans for subjecting the spot position signal to a second processing bya traverse loop filter, and outputting the signal to the second movingmeans; and system operation control means for operating the firstcontrol means to perform a phase-lag compensation and a phase-leadcompensation by the phase compensation loop filter to the spot positionsignal, thereafter switching the spot position signal to the trackingerror signal to perform the phase-lag compensation and the phase-leadcompensation to the tracking error signal, and operating the secondcontrol means after operating the first control means. Therefore, evenwhen the self-weight dislocation occurs in the objective lens, thetraverse control using the spot position signal can be performed withstability. Besides, a loop filter can be commonly used for performingthe phase-lead compensation and the phase-lag compensation in performingthe spot position control and the tracking error control. Therefore, anoptical disc apparatus which is considerable effective in practical use,particularly as a portable optical disc apparatus, can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a block diagram illustrating an optical disc apparatusaccording to a first embodiment of the present invention.

[0017]FIG. 2 is a waveform chart showing operations according to thefirst embodiment.

[0018]FIG. 3 is a flowchart showing processings by a controller 116 inthe first embodiment.

[0019]FIG. 4 is a block diagram illustrating an optical disc apparatusaccording to a second embodiment of the present invention.

[0020]FIG. 5 is a waveform chart showing operations according to thesecond embodiment.

[0021]FIG. 6 is a flowchart showing processings by a controller 116 inthe second embodiment.

[0022]FIG. 7 is a block diagram illustrating an optical disc apparatusaccording to a third embodiment of the present invention.

[0023]FIG. 8 is a waveform chart showing operations according to thethird embodiment.

[0024]FIG. 9 is a flowchart showing processings by a controller 604 inthe third embodiment.

[0025]FIG. 10 is a chart showing waveforms of a correction signal in thethird embodiment.

[0026]FIG. 11 is a block diagram illustrating an optical disc apparatusaccording to a fourth embodiment of the present invention.

[0027]FIG. 12 is a waveform chart showing operations according to thefourth embodiment.

[0028]FIG. 13 is a flowchart showing processings by a controller 1002 inthe fourth embodiment.

[0029]FIG. 14 is a waveform chart showing operations according to aconventional optical disc apparatus.

[0030]FIG. 15 are diagrams showing loop characteristics of spot positioncontrol, FIG. 15(a) being a diagram showing open-loop gaincharacteristics without a spot position loop filter, FIG. 15(b) being adiagram showing gain characteristics of the spot position loop filter,and FIG. 15(c) being a diagram showing open-loop characteristics of thewhole spot position control.

[0031]FIG. 16 is a block diagram illustrating an optical disc apparatusaccording to another example of the first embodiment.

[0032]FIG. 17 are diagrams showing loop characteristics of spot positioncontrol, FIG. 17(a) being a diagram showing open-loop gaincharacteristics without a spot position loop filter, FIG. 17(b) being adiagram showing gain characteristics of the spot position loop filter,and

[0033]FIG. 17(c) being a diagram showing open-loop characteristics ofthe whole spot position control.

[0034]FIG. 18 are diagrams showing loop characteristics of spot positioncontrol, FIG. 18(a) being a diagram showing open-loop gaincharacteristics without a spot position loop filter, FIG. 18(b) being adiagram showing gain characteristics of the spot position loop filter,FIG. 18(c) being a diagram showing open-loop characteristics of thewhole spot position control, FIG. 18(d) being a diagram showing gaincharacteristics of a changed spot position loop filter, and FIG. 18(e)being a diagram showing open-loop characteristics of the whole changedspot position control.

[0035]FIG. 19 are diagrams showing loop-characteristics of spot positioncontrol, FIG. 19(a) being a diagram showing open-loop gaincharacteristics without a spot position loop filter, FIG. 19(b) being adiagram showing a gain characteristics of the spot position loop filter,FIG. 19(c) being a diagram showing open-loop characteristics of thewhole spot position control, FIG. 19(d) being a diagram showing gaincharacteristics of a spot position loop filter, and FIG. 19(e) being adiagram showing open-loop characteristics of the whole spot positioncontrol.

[0036]FIG. 20 are diagrams showing loop characteristics of spot positioncontrol, FIG. 20(a) being a diagram showing open-loop gaincharacteristics without a spot position loop filter 111, FIG. 20(b)being a diagram showing gain characteristics of the spot position loopfilter 111, FIG. 20(c) being a diagram showing open-loop characteristicsof the whole spot position control, FIG. 20(d) being a diagram showingopen-loop phase characteristics without the spot position loop filter111 and phase characteristics of the spot position loop filter 111, inthe spot position control loop, and FIG. 20(e) being a diagram showingphase characteristics of the whole spot position control loop.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Hereinafter, embodiments of the present invention will bedescribed with reference to the drawings.

[0038] Embodiment 1.

[0039] A first embodiment of the present invention will be describedwith reference to FIGS. 1, 2, 3, and 15.

[0040] The first embodiment corresponds to claims 1, 5, 6 and 7, and itallows the traverse control by executing spot position controlbeforehand, even when the self-weight dislocation occurs in an objectivelens in a radial direction of an optical disc.

[0041]FIG. 1 is a block diagram illustrating an optical disc apparatusaccording to the first embodiment. FIG. 2 is a waveform chart showingoperations according to the first embodiment. FIG. 3 is a flowchartshowing processings by a controller 116 in the first embodiment.

[0042] In FIG. 1, reference numeral 100 denotes a mechanism unit. Inthis mechanism unit 100, numeral 101 denotes an optical disc havingconcentric or helical tracks, on or from which information is recordedor reproduced. Numeral 102 denotes an optical head, internallycontaining an objective lens 103 as converging means for converging alight spot on the optical disc 101, a tracking actuator 104 as firstmoving means, for moving the objective lens 103 in a radial direction ofthe optical disc 101, and a light receiving element 105 for converting alight reflected on the optical disc 101 into an electric signal. Numeral106 denotes a traverse motor as second moving means, for moving theoptical head 102 in a radial direction of the optical disc in accordancewith an output from a switch circuit 114 which is described later.

[0043] In addition, numeral 107 denotes a control unit. In this controlunit 107, numeral 108 denotes a tracking error detection circuit forgenerating a tracking error signal which indicates positionaldisplacement between the track on the optical disc 101 and the lightspot, from an output of the light receiving element 105. Numeral 109denotes a tracking loop filter which receives the tracking error signalas input, and outputs the signal after performing phase compensationprocessing. Numeral 110 denotes a spot position detection circuit asspot position detection means, for generating a spot position signalwhich indicates a positional difference between the center of theoptical head and the light spot on the light receiving element 105, froman output of the light receiving element 105. Numeral 111 denotes a spotposition loop filter which receives the spot position signal as input,and outputs the signal after performing the phase compensationprocessing. Numeral 112 denotes a selection circuit for selecting eitherof an output of the tracking loop filter 109 and an output of the spotposition loop filter 111, in accordance with an instruction of acontroller 116, which is described later, and supplying the selectedoutput to the tracking actuator 104. Numeral 113 denotes a traverse loopfilter which receives the spot position signal as input, and outputs thesignal after performing the phase compensation processing. Numeral 114denotes a switch circuit for switching on/off an output of the traverseloop filter 113, in accordance with an instruction of the controller116, which is described later. Numeral 116 denotes a controller whichreceives a detection result of the spot position detection circuit 110as input, and controls the selection circuit 112 and the switch circuit114, on the basis of the detection result. This controller 116constitutes first control means, together with the selection circuit112. In addition, the controller 116 constitutes second control means,together with the switch circuit 114. Further, the controller 116constitutes system operation control means for operating the secondcontrol means after operating the first control means.

[0044] Next, a description is given of an operation of the soconstructed optical disc apparatus according to the first embodiment.

[0045] When the objective lens 103 experiences gravity in a radialdirection of the optical disc, the objective lens 103 is displaced dueto the self-weight dislocation. The spot position detection circuit 110generates a spot position signal on the basis of the displacement amountof the light spot displaced from the center of the light receivingelement 105, and outputs the signal. At an initial timing when nocontrol is imposed (see 21 in FIG. 2), the spot position signal has anoffset of a value A (A≠0). The optical disc apparatus of the firstembodiment controls the position of the objective lens 103 in thefollowing procedure so as to make the value of the spot position signalapproximately “0”.

[0046] The controller 116 opens the switch circuit 114, switches theselection circuit 112 to the side for the spot position loop filter 111in a state where driving of the traverse motor 106 is stopped, andsupplies an output from the spot position loop filter 111 to thetracking actuator 104 (step S301 in FIG. 3). The spot position loopfilter 111 receives the spot position signal, and the spot position loopfilter 111 controls the position of the objective lens 103 via thetracking actuator 104, so as to have approximately “0” spot positionsignal, i.e., to position the light spot at the center of the lightreceiving element 105. Hereinafter, this control which is newlyperformed is referred to as “spot position control”. The displacementamount of the objective lens 103 due to the self-weight dislocation isdetermined by hardness of a resilient support supporting the objectivelens 103 and weight of the objective lens 103. Further, a time periodfrom when the spot position control for the determined displacementamount is started until the spot position signal has a value ofapproximately “0” is determined by characteristics of a servo loop whichis constituted by the tracking actuator 104, the spot position detectioncircuit 110 and the spot position loop filter 111, and it can bepreviously calculated. The controller 116 waits for the previouslycalculated time period (see 22 in FIG. 2) until the spot position signalhas a value of approximately “0” (step S302 in FIG. 3), switches theselection circuit 112 to the side for the tracking loop filter 109, andcloses the switch circuit 114 (step S303 in FIG. 3). When the selectioncircuit 112 is switched to the side of the tracking loop filter 109, thetracking error signal detected by the tracking error detection circuit108 is subjected to phase compensation processing by the tracking loopfilter 109, then supplied to the tracking actuator 104, and thereby thelight spot is controlled so as to follow the tracks on the optical discthrough the objective lens 103. When the switch circuit 114 is closed,the spot position signal which is subjected to the phase compensation bythe traverse loop filter 113 is supplied to the traverse motor 106, andthereby the optical head 102 is moved to a position where the spotposition signal has an approximately “0” value. At starting of thetraverse control (see 23 in FIG. 2), since the spot position signal hasan approximately “0” value because of the spot position control executedbeforehand, the traverse driving signal does not oscillate unlike thatshown in the prior art. Therefore, the traverse control having a controlobject of positioning the light spot near the center of the lightreceiving element 105 (≈ near the center of the optical head 102) can bestarted with stability.

[0047] A more detailed description is given of the spot position loopfilter 111. The spot position control in the first embodiment isrealized by a control loop comprising the tracking actuator 104, thespot position detection circuit 110, the spot position loop filter 111,and the selection circuit 112. This control system is a closed-loopfeedback control system. FIG. 15(a) schematically shows gaincharacteristics of elements except the spot position loop filter 111. Asshown in FIG. 15(a), it does not have frequency characteristics atfrequencies lower than a primary resonance frequency f0 and hascharacteristics that the gain is decreased by −40 dB/dec at frequencieshigher than f0. The characteristics shown in FIG. 15(a) are seen in acontrol target having a structure for supporting the objective lens 103by a resilient support such as a spring. When the control is to beperformed to such a control target, it is effective to perform phasecompensation by a differential operation, i.e., phase-lead compensation,in addition to the phase-lag compensation. That is, as shown in FIG.15(b), when the spot position loop filter 111 has characteristics of 20dB/dec (phase-lead compensation) and −20 dB/dec (phase-lag compensation)and the inclination in the vicinity of the gain crossover frequency fcin the whole open-loop characteristics (see FIG. 15(c)) is −20 dB/dec,the control system can be stabilized.

[0048] Since the control target is the same tracking actuator 104,characteristics required in the tracking loop filter 109 areapproximately similar to those shown in FIG. 15(b). Therefore, loopfilters can be commonly used by using the spot position loop filter 111and the tracking loop filter 109 exclusively.

[0049]FIG. 16 is a block diagram illustrating an optical disc apparatusaccording to the first embodiment, which is constructed as describedabove. In FIG. 16, numerals 100-108, 110, 113, and 114 denote elementsof the same numerals as those shown in FIG. 1. Numeral 120 denotes aselection circuit for selecting either of an output from the trackingerror detection circuit 108 and an output from the spot positiondetection circuit 110 in accordance with an instruction of a controller116, which is described later, and supplying the selected output to aphase compensation loop filter 121. Numeral 121 denotes a phasecompensation loop filter which has V-shaped gain characteristics andperforms phase-lag compensation in a low band and phase-leadcompensation in a high band to the tracking error signal or spotposition signal selected by the selection circuit 120. Numeral 116denotes a controller which receives a detection result of the spotposition detection circuit 110 as an input, and controls the selectioncircuit 120 and the switch circuit 114 on the basis of the detectionresult. This controller 116 constitutes first control means togetherwith the selection circuit 120. In addition, the controller 116constitutes second control means together with the selection circuit120. Further, this controller 116 constitutes system operation controlmeans for operating the first control means, and thereafter operatingthe second control means.

[0050] According to the structure shown in FIG. 16, a loop filter havingthe same gain characteristics as the spot position loop filter 111 asshown in FIG. 15(b) is provided as the phase compensation loop filter116. Then, the controller 116 executes the control such that theselection circuit 120 inputs the spot position signal initially to thephase compensation loop filter 116, and switches the input for the phasecompensation loop filter 116 to the tracking error signal when the spotposition control is almost completed. Accordingly, the spot positionloop filter 111 and the tracking loop filter 119 in FIG. 1 can becommonly used.

[0051] When sensitivity of the spot position signal and the trackingerror signal is extremely different from each other, the characteristicsof the spot position loop filter 111 and the tracking loop filter 109should be differentiated. In a CD or MD, while the track pitch is 1.6μm, the movable range of the objective lens 103 is hundreds μm.Therefore, in the case of the same signal amplitude, the sensitivity ofthe spot position signal is less than one hundredth of that of thetracking error signal. When the open-loop characteristics shown in FIG.15(c) are to be realized in both of the tracking control loop and thespot position control loop, the gain of the spot position loop filter111 is required to be more than one hundred times as large as that ofthe track position loop filter 109, and this is not realistic.Therefore, in such a case, it is effective to make the gain crossoverfrequency fc lower than the primary resonance frequency f0 and make thespot position loop filter have the characteristics shown in FIG. 17(b).In this case, as shown in FIG. 17(c) showing the whole open-loopcharacteristics, the gain characteristics in the vicinity of the gaincrossover frequency can be set at −20 dB/dec. For example in astationary optical disc apparatus, since the direction of theself-weight dislocation is fixed, the control band of the spot positioncontrol is not required to be enhanced and therefore this method isparticularly effective. Further, advantageously, this method can beeasily realized by a simple filter of −20 dB/dec.

[0052] When the spot position loop filter 111 is constituted using thefilter of −20 dB/dec, attention should be given to a gain at the primaryresonance frequency f0, i.e., a resonance point. When the gain at theresonance point is large, a part having the gain more than 0 dB occursat frequencies higher than the gain crossover frequency fc as shown inFIG. 18(c), whereby the spot position control becomes unstable. To solvethis problem, it is effective to make the spot position loop filter 111have the characteristics of reducing the gain at high frequencies, asshown in FIG. 18(d). Accordingly, the open-loop gain in excess of 0 dBat the resonance point can be avoided as shown in FIG. 18(e), wherebythe spot position control can be stabilized.

[0053] When the spot position control is executed using the spotposition signal having a low sensitivity, by using the phase-leadcompensation as described with reference to FIG. 17, it is effective tostart the phase-lead compensation from a frequency lower than theprimary resonance frequency f0, as shown in FIG. 19(d). By doing so,even when the open-loop gain exceeds 0 dB at the resonance point, aphase margin can be secured, thereby stabilizing the spot positioncontrol.

[0054] With regard to this point, a detailed description is given withreference to the drawings. FIG. 20(a) shows open-loop gaincharacteristics without the spot position loop filter 111, FIG. 20(b)shows gain characteristics of the spot position loop filter 111, FIG.20(c) shows open-loop gain characteristics of the whole spot positioncontrol loop, FIG. 20(d) shows open-loop phase characteristics withoutthe spot position loop filter 111 and phase characteristics of the spotposition loop filter in the spot position control loop, and FIG. 20(e)shows phase characteristics of the whole spot position control loop. Inthe figures, f0 denotes a primary resonance frequency and fB denotes afrequency at which phase-lead compensation is started. Numeral 181denotes open-loop phase characteristics without the spot position loopfilter 111. Numeral 182 denotes phase characteristics of the spotposition loop filter. Numeral 183 denotes a phase margin for the spotposition control.

[0055] The phase characteristic 181 of elements except the spot positionloop filter 111 is 0 degree at a frequency band lower than the primaryresonance frequency f0 and thereafter abruptly decreased up to −180degree, passing through a point of about −90 degree at the primaryresonance frequency f0. On the other hand, the phase characteristic 182of the spot position loop filter 111 is about 0 degree at the phase-leadcompensation starting frequency fB, −90 degree at a frequency bandsufficiently lower than fB, and +90 degree at a frequency bandsufficiently higher than fB. The phase characteristic of the whole loopcan be obtained by adding these characteristics, which is shown in FIG.20(e). That is, the phase-lead compensation is started from thefrequency band lower than that in the vicinity of the primary resonancefrequency f0 where the phase is abruptly decreased to −180 degree (i.e.,phase margin of 0), whereby the phase margin 183 can be secured and thespot position control can be stabilized.

[0056] While this method slightly complicates the structure of the spotposition loop filter 111, it is resistant to variation in the open-loopgain at the resonance point, more specifically, variation in the gain ofthe tracking actuator 104 at the primary resonance frequency f0, and itcan enhance the gain crossover frequency fc with relative to that in acase where the phase-lead compensation is not performed, therebyobtaining a quicker response. Generally, the gain of the trackingactuator 104 at the primary resonance frequency f0 varies according tothe weight of the objective lens 103, hardness of a spring whichsupports the objective lens, hardness of adhesive which is used forfixing the spring to the optical head 102, and the like. Therefore, itis difficult to reduce the variations. Accordingly, the present methodwhich can accommodate the variations is effective in practical use. Inaddition, by providing a mechanism for switching the frequency where thephase-lead compensation is started, for example by switching a tap tothe spot position loop filter 111, the spot position loop filter 111 canbe commonly used with the tracking loop filter 109. In this case, thecomplicated structure of the spot position loop filter 111 presents noproblem any more.

[0057] Here, it is desirable that the switching of the selection circuit112 to the side for the tracking loop filter 109 is completed before orsimultaneously with the closing of the switch circuit 114. As describedabove, the output of the tracking loop filter 109 has a function offixing the position of the light spot to the track on the optical disc101. However, when the switch circuit 114 is closed in a state wherethis control is not performed, while the optical head 102 is movedaccording to the spot position signal, the light receiving element 105,the tracking actuator 104 and the objective lens 103 are moved togetherwith the optical head 102. Therefore, the spot position signal is notreduced and thus there is a risk of the optical head 102 mechanicallycontinuing to move until the movable limit. In this first embodiment,since the light spot is fixed to the track as described above, thetraverse control using the spot position signal is realized.

[0058] As described above, the apparatus according to the firstembodiment performs the spot position control, corrects the self-weightdislocation of the objective lens 103 occurring in the radial directionof the disc, and thereafter performs the traverse control. Thereby, evenin a case where the self-weight dislocation occurs according to theposition of the apparatus, the traverse control using the spot positionsignal can be performed with stability.

[0059] Embodiment 2.

[0060] Hereinafter, a second embodiment of the present invention will bedescribed with reference to FIGS. 4, 5, and 6.

[0061] The second embodiment corresponds to claims 2, 5, and 6.

[0062] In the first embodiment, the fixed time previously calculated(see 22 in FIG. 2) is set as the time period from the starting of thespot position control until the starting of the traverse control.However, the hardness of the resilient support which supports theobjective lens 103 or the sensitivity of the tracking actuator 104 forinput signals usually have variations. Therefore, in order to operatethe apparatus with stability on all conditions, a time period includinga longer margin should be set to satisfy the worst condition.

[0063] In this second embodiment, a monitoring circuit 115 is providedas shown in FIG. 4 and the spot position loop filter 111 is operatedduring the shortest possible time when oscillations do not occur at thestarting of the traverse control, whereby the whole control time can beoptimized.

[0064]FIG. 4 is a block diagram illustrating an optical disc apparatusaccording to the second embodiment. FIG. 5 is a waveform chart showingoperations according to the second embodiment. FIG. 6 is a flowchartshowing processings by a controller 116 in the second embodiment.

[0065] In FIG. 4, reference numeral 100 denotes a mechanism unit. Inthis mechanism unit 100, numeral 101 denotes an optical disc havingconcentric or helical tracks, on or from which information is recordedor reproduced. Numeral 102 denotes an optical head, which internallycontains an objective lens 103 as converging means for converging alight spot on the optical disc 101, a tracking actuator 104 as firstmoving means, for moving the objective lens 103 in a radial direction ofthe optical disc, and a light receiving element 105 for converting alight reflected on the optical disc 101 into an electric signal. Numeral106 denotes a traverse motor as second moving means, for moving theoptical head 102 in a radial direction of the optical disc, inaccordance with an output from a switch circuit 114, which is describedlater.

[0066] Numeral 107 denotes a control unit. In this control unit 107,numeral 108 denotes a tracking error detection circuit for generating atracking error signal which indicates position displacement between thetrack on the optical disc 101 and the light spot, from an output of thelight receiving element 105. Numeral 109 denotes a tracking filter whichreceives the tracking error signal as input, and outputs the signalafter performing phase compensation processing. Numeral 110 denotes aspot position detection circuit as spot position detection means, forgenerating a spot position signal which indicates a positionaldifference between the center of the optical head and the light spot onthe light receiving element 105, from the output of the light receivingelement 105. Numeral 111 denotes a spot position loop filter whichreceives the spot position signal as input, and outputs the signal afterperforming the phase compensation processing. Numeral 112 denotes aselection circuit for selecting either of an output from the trackingloop filter 109 and an output from the spot position loop filter 111, inaccordance with an instruction of a controller 116, which is describedlater, and supplying the selected output to the tracking actuator 104.Numeral 113 denotes a traverse loop filter which receives the spotposition signal as input, and outputs the signal after performing thephase compensation processing. Numeral 114 denotes a switch circuit forswitching on or off the output of the traverse loop filter 113, inaccordance with an instruction of the controller 116 which is describedlater. Numeral 115 denotes a monitoring circuit as spot position signalmonitoring means, for judging the size of the spot position signal.Numeral 116 denotes a controller which receives a monitor result of themonitoring circuit 115 as input, and controls the selection circuit 112and the switch circuit 114 on the basis of the monitor result. Thiscontroller 116 constitutes first control means together with theselection circuit 112. In addition, the controller 116 constitutessecond control means together with the switch circuit 114. Further, thecontroller 116 constitutes system operation control means for operatingthe first control means when it receives a signal indicating that thespot position control is statically determined from the monitoringcircuit 115, and operating the second control means after orsimultaneously with the operation of the first control means.

[0067] Next, a description is given of an operation of the soconstructed optical disc apparatus according to the second embodiment.

[0068] When the objective lens 103 experiences gravity in a radialdirection of the optical disc 101, the objective lens is displaced dueto the self-weight dislocation. The spot position detection circuit 110generates a spot position signal in accordance with the displacementamount of the light spot from the center of the light receiving element105, and outputs the spot position signal. At an initial timing when nocontrol is imposed (see 41 in FIG. 5), the spot position signal has anoffset of a value A (A≠0). The controller 116 opens the switch circuit114 thereby stopping the driving of the traverse motor 106, and switchesthe selection circuit 112 to the side for the spot position loop filter111 to make the value of the spot position signal approximately “0”,thereby executing the spot position control (step S501 in FIG. 6). Then,the controller 116 receives the input from the monitoring circuit 115(step S502 in FIG. 6). The monitoring circuit 115 monitors whether thevalue of the spot position signal goes into a previously determinedrange (see 42 in FIG. 5), and judges that the spot position control isstatically determined at a timing when the value goes into thepredetermined range. The controller 116 judges whether the output of themonitoring circuit 115 indicates that the spot position control isstatically determined (step S503 in FIG. 6). When it is judged that theoutput indicates the static determination, the selection circuit 112 isswitched to the side for the tracking loop filter 109 as well as theswitch circuit 114 is closed (see 43 in FIG. 5 and step S504 in FIG. 6).At this timing, since the spot position signal has a sufficiently smallvalue, the traverse control is performed without being oscillated. Inaddition, the execution time of the spot position control (see 44 inFIG. 5) can be shortened with relative to a case where the monitoringcircuit 115 is not used. When it is judged that the spot positioncontrol is not statically determined, the controller 116 successivelyreceives the output from the monitoring circuit 115, and judges whetherit is statically determined.

[0069] It is desirable that a value compared with the spot positionsignal, which is used by the monitoring circuit 115 to judge the staticdetermination of the spot position control is set to have a valuesmaller than an operation unit of the traverse motor 106. When thetraverse motor 106 is constituted by a motor with brush, this traversemotor 106 often operates with “Cogging unit” as a unit. For example, ina structure where the optical head 102 is moved 40 μm per 1 Cogging,assuming that the spot position control is statically determined at atiming when the spot position signal goes into a range of about ±40 μm,this is equivalent to a situation where the error is approximately “0”in the traverse control performed later. Therefore, the turbulence inthe traverse control can be avoided.

[0070] As described above, the apparatus according to the secondembodiment includes the monitoring circuit 115 in addition to theapparatus of the first embodiment and compares the spot position signalwith the previously set range, whereby the spot position control can beensured with stability. Further, the apparatus can optimize and minimizethe time period for executing the spot position control (see 44 in FIG.5) for each execution of the traverse control, thereby obtaining thestable and high-speed operation of the optical disc apparatus.

[0071] In this second embodiment, the monitoring circuit 115 judges thatthe spot position control is statically determined, at a timing when thespot position signal goes into the predetermined range (see 42 in FIG.5). However, any method, such as a method for detecting that a timeperiod when the spot position signal is within a predetermined range islonger than a predetermined time period, or a method for detecting thatan average of the spot position signals in a predetermined time periodis smaller than a predetermined value, can be utilized, as long as themethod substantially detects that the size of the spot position signalis reduced and judges the static determination of the spot positioncontrol.

[0072] Embodiment 3.

[0073] A third embodiment of the present invention will be describedwith reference to FIGS. 7, 8, 9, and 10.

[0074] The third embodiment corresponds to claim 3. This thirdembodiment controls the size of a spot position signal input to atraverse loop filter also in an initial state, thereby obtaining thestability of the traverse control.

[0075]FIG. 7 is a block diagram illustrating an optical disc apparatusaccording to the third embodiment. FIG. 8 is a waveform chart showingoperations according to the third embodiment. FIG. 9 is a flowchartshowing processings by a controller 604. FIG. 10 is a diagram showingwaveforms of correction signals in the third embodiment.

[0076] In FIG. 7, numerals 100-114 denote elements of the same referencenumerals as those shown in FIG. 1. Numeral 601 denotes a correctionsignal generation circuit as correction signal generation means, forgenerating a correction signal for correcting a spot position signal.Numeral 602 denotes a subtracter as subtracting means for subtractingthe correction signal from the spot position signal (hereinafterreferred to as “before-correction spot position signal) output from thespot position detection circuit 110. The spot position signal input tothe traverse loop filter 113 after being corrected by using thecorrection signal is hereinafter referred to as “after-correction spotposition signal”. Numeral 603 denotes a second switch circuit forswitching on/off the supply of the signal to the tracking actuator 104,in accordance with an instruction of a controller 604 which is describedlater. Numeral 604 denotes a controller for controlling the switchcircuit 114, the second switch circuit 603, and the correction signalgeneration circuit 601. This controller 604 constitutes second controlmeans together with the switch circuit 114. The second control meanssubjects the output of the subtracter 602 to the processing by thetraverse loop filter 113, and outputs the output to the traverse motor106 as the second moving means, via the switch circuit 114.

[0077] Next, a description is given of an operation of the soconstructed optical disc apparatus according to the third embodiment.

[0078] When the objective lens 103 experiences gravity in a radialdirection of the optical disc 101, the objective lens 103 is displaceddue to the self-weight. The spot position signal has an offset of apredetermined value A (A≠0), as shown by numeral 71 in FIG. 8. Thecontroller 604 controls the correction signal generation circuit 601 tostore the value of the spot position signal at that time in prior to theexecution of the traverse control and output the value as the correctionsignal (see 72 in FIG. 8 and step S801 in FIG. 9). The subtracter 602subtracts the correction signal from a before-correction spot positionsignal output by the spot position detection circuit 110. Accordingly,an after-correction spot position signal output from the subtracter 602has a value of approximately “0” (see 73 in FIG. 8). Then, thecontroller 604 closes the second switch circuit 603 thereby supplyingthe output of the tracking loop filter 109 to the tracking actuator 104,controls the light spot to follow the track on the optical disc 101,further closes the switch circuit 114 thereby supplying the output ofthe traverse loop filter 113 to the traverse motor 106, and therebymoves the optical head 102 so as to locate the light spot in thevicinity of the center of the optical head 102 (step S802 in FIG. 9). Atthis time, since the after-correction spot position signal input to thetraverse loop filter 113 is approximately “0”, the output from thetraverse loop filter 113 is also approximately “0”. Therefore, thetraverse motor 106 scarcely operates.

[0079] Then, the controller 604 instructs the correction signalgeneration circuit 601 to gradually increase the correction signal up tonear “0” (step S803 in FIG. 9), and waits for the completion of thecorrection signal changing by the correction signal generation circuit601 (step S804 in FIG. 9). When the correction signal is changed, adifference is generated between the before-correction spot positionsignal and the correction signal. Since the traverse loop filter 113 andthe traverse motor 106 operate so as to make this difference, i.e., theafter-correction spot position signal “0”, the before-correction spotposition signal is changed so as to have almost the same waveform asthat of the correction signal (see 74 in FIG. 8). When the correctionsignal which is to be output has “0” value, the correction signalgeneration circuit 601 keeps the correction signal in “0” (see 75 inFIG. 8), and notifies to the controller 604 of the completion of thecorrection signal changing. When the controller 604 receives thenotification of the completion of the correction signal changing, itcompletes the starting of the traverse control.

[0080] The spot position signal can be changed by the correction signallinearly with a predetermined time constant (see 91 in FIG. 10) orsinusoidally (see 92 in FIG. 10). However, the latter is more desirablebecause the spot position control can be followed more smoothly in thatcase. Further, it is desirable that the changing speed of the correctionsignal is lower than the control band of the traverse control.Accordingly, the traverse control can follow the change in the spotposition signal with stability.

[0081] As described above, the apparatus according to the thirdembodiment stores the spot position signal at the starting of thetraverse control, subtracts the stored signal as a correction signalfrom the spot position signal, and supplies the obtained difference tothe traverse loop filter 113. Therefore, the input to the traverse loopfilter 113 can be always kept in a small value, whereby the unstablenessin the traverse control due to the large initial error signal can beavoided.

[0082] In this third embodiment, the correction signal is subtractedusing the subtracter 602. However, any unit, such as a unit for holdinga correction signal after performing the inversion and using the adderto subtract the correction signal from the before-correction spotposition signal, can be utilized as long as it can substantiallysubtract the correction signal from the before-correction spot positionsignal.

[0083] Embodiment 4.

[0084] A fourth embodiment of the present invention will be describedwith reference to FIGS. 11, 12, and 13.

[0085] The fourth embodiment corresponds to claim 4. In this embodiment,a loop gain of the traverse control is set to have a small value at thestarting of the traverse control, and thereafter the gain is changed tohave a desired value, thereby stabilizing the operation at the startingof the traverse control.

[0086]FIG. 11 is a block diagram illustrating an optical disc apparatusaccording to the fourth embodiment. FIG. 12 is a waveform chart showingoperations according to the fourth embodiment. FIG. 13 is a flowchartshowing processings by a controller 1002.

[0087] In FIG. 11, numerals 100-114 denote the elements of the samereference numerals as those shown in FIG. 1. In addition, numeral 603denotes the element of the same reference numeral as that shown in FIG.7. Numeral 1001 denotes a coefficient multiplier for coefficientmultiplying an output from the traverse loop filter 113, and outputtingthe same. Numeral 1002 denotes a controller for controlling the switchcircuit 114, the coefficient multiplier 1001, and the second switchcircuit 603. This controller 1002 constitutes second control meanstogether with the coefficient multiplier 1001 and the switch circuit114. The second control means subjects the spot position signal to theprocessing by the traverse loop filter 113 and outputs the processedsignal to the traverse motor 106 as the second moving means, as well ascontrols a coefficient of the coefficient multiplier 1001 at thestarting of the operation to have a value smaller than that at thenormal operation.

[0088] Hereinafter, an operation of the optical disc apparatus accordingto the fourth embodiment will be described.

[0089] When gravity is applied to the objective lens 103 in a radialdirection of the optical disc 101, the objective lens 103 is displacedfrom the center of the optical head 102 due to the self-weight.Accordingly, the spot position signal has an offset of a value A (see1101 in FIG. 12). The controller 1002 opens the switch circuit 114 andthe second switch circuit 603, and stops the driving of the trackingactuator 104 and the traverse motor 106 (step S1201 in FIG. 13). Next,the controller 1002 sets a coefficient of the coefficient multiplier1001 at B times (0<B<1), for example, 0.1 times (see 1102 in FIG. 12 andstep S1202 in FIG. 13). Then, the controller closes the switch circuit114 and the second switch circuit 603, supplies the output of thetracking loop filter 109 to the tracking actuator 104, and supplies theoutput of the traverse loop filter 113, which is coefficient multipliedby the coefficient multiplier 1001, to the traverse motor 106, therebyexecuting the tracking control and the traverse control (step S1203 inFIG. 13). In this case, the spot position signal input to the traverseloop filter 113 remains having the value A. However, since the value Bis set in the coefficient multiplier 1001, the voltage supplied to thetraverse motor 106 is reduced with relative to a case without thecoefficient multiplier 1001. Therefore, the operation of the opticalhead 102 by the traverse motor 106 is slowed. The controller 1002 makesthe coefficient of the coefficient multiplier 1001 closer to “1” withthe lapse of time (see 1103 in FIG. 12 and step S1204 in FIG. 13). Inthis way, the coefficient of the coefficient multiplier 1001 isinitially set to have a smaller value in the state where the spotposition signal is large and the coefficient is increased with the lapseof time, whereby the optical head 102 can be moved smoothly. Thecontroller 1002 judges that the coefficient has a value of “1”, andcompletes rising processing of the traverse control in the state of thecoefficient of “1” (step S1205 in FIG. 13).

[0090] As described above, the apparatus according to the fourthembodiment includes the coefficient multiplier 1001, sets thecoefficient of the coefficient multiplier to have a smaller value at thestarting of the traverse control to decrease the loop gain of thetraverse control, and gradually increases the coefficient, therebyobtaining the desired gain. Therefore, the operation at the starting ofthe traverse control can be stabilized with a simple structure using thecoefficient multiplier, without providing the new control system whichperforms the tracking control by the spot position signal as describedin the first and second embodiments.

[0091] In this fourth embodiment, the coefficient multiplier 1001 isprovided in a next stage of the traverse loop filter 113. However, thiscan be provided between the spot position detection circuit 110 and thetraverse loop filter 113, or between the switch circuit 114 and thetraverse motor 106. Or, any unit including a unit for changing the gainof the traverse loop filter 113 can be used as long as it changes theloop gain of the servo loop constituting the traverse control.

[0092] Further, in the fourth embodiment, the example where thecoefficient of the coefficient multiplier 1001 is changed linearly isdescribed. However, as similar to the correction signal in the thirdembodiment, the coefficient can be changed in any manner including amanner of changing the coefficient sinusoidally, as long as thecoefficient is set to have a small value at first and finally have thedesired gain for the traverse control loop.

[0093] In the foregoing description, the spot position signal isgenerated from the output of the light receiving element 105 in theoptical head 102. However, a lens position sensor for detecting aposition of the lens can be provided separately from the light receivingelement. In such a structure, the lens position can be used as a spotposition.

What is claimed is:
 1. An optical disc apparatus which applies a light spot to an optical disc, thereby to record or reproduce information on or from the optical disc, comprising: first moving means for moving the light spot applied to the optical disc, in a radial direction of the optical disc; an optical head having converging means for converging the light spot on the optical disc; spot position detection means for generating a spot position signal which indicates a positional difference in a radial direction of the optical disc, between a center of the optical head and the light spot on the optical head; first control means for subjecting the spot position signal to a first processing by a spot position loop filter, and outputting the spot position signal to the first moving means; second moving means for moving the optical head in a radial direction of the optical disc; second control means for subjecting the spot position signal to a second processing by a traverse loop filter, and outputting the spot position signal to the second moving means; and system operation control means for operating the first control means, and thereafter operating the second control means.
 2. An optical disc apparatus which applies a light spot to an optical disc, thereby to record or reproduce information on or from the optical disc, comprising: first moving means for moving the light spot applied to the optical disc, in a radial direction of the optical disc; an optical head having converging means for converging the light spot on the optical disc; spot position detection means for generating a spot position signal which indicates a positional difference in a radial direction of the optical disc, between a center of the optical head and the light spot on the optical head; first control means for subjecting the spot position signal to a first processing by a spot position loop filter, and outputting the spot position signal to the first moving means; second moving means for moving the optical head in a radial direction of the optical disc; second control means for subjecting the spot position signal to a second processing by a traverse loop filter, and outputting the spot position signal to the second moving means; spot position signal monitoring means for receiving the spot position signal as an input, and outputting a first signal which indicates that the spot position signal comes to a value smaller than a prescribed value; and system operation control means for operating the first control means when the first signal is input, and operating the second control means after or simultaneously with the operation of the first control means.
 3. An optical disc apparatus which applies a light spot to an optical disc, thereby to record or reproduce information on or from the optical disc, comprising: first moving means for moving the light spot applied to the optical disc, in a radial direction of the optical disc; an optical head having converging means for converging the light spot on the optical disc; spot position detection means for generating a spot position signal which indicates a positional difference in a radial direction of the optical disc, between a center of the optical head and the light spot on the optical head; correction signal generation means for receiving the spot position signal as an input, and generating a correction signal for correcting the spot position signal; subtracting means for subtracting the correction signal from the spot position signal; second moving means for moving the optical head in a radial direction of the optical disc; and second control means for subjecting an output from the subtracting means to a processing by a traverse loop filter, and outputting the output to the second moving means.
 4. An optical disc apparatus which applies a light spot to an optical disc, thereby to record and reproduce information on or from the optical disc, comprising: first moving means for moving the light spot applied to the optical disc, in a radial direction of the optical disc; an optical head having converging means for converging the light spot on the optical disc; spot position detection means for generating a spot position signal which indicates a positional difference in a radial direction of the optical disc, between a center of the optical head and the light spot on the optical head; second moving means for moving the optical head in a radial direction of the optical disc; second control means for subjecting the spot position signal to a processing by a traverse loop filter, and outputting the spot position signal to the second moving means; and a coefficient multiplier for reducing a coefficient for the control by the second control means to a value smaller than that in a normal operation time, at starting of the operation of the second control means.
 5. The optical disc apparatus of claim 1 or 2 wherein the first processing subjected by the first control means is a phase-lag compensation.
 6. The optical disc apparatus of claim 5 wherein the first processing subjected by the first control means includes compensation for reducing an open-loop gain at a primary resonance frequency of the first moving means, in addition to the phase-lag compensation.
 7. The optical disc apparatus of claim 1 or 2 wherein the first processing subjected by the first control means is a phase-lead compensation and a phase-lag compensation, and the phase-lead compensation is started from a frequency lower than a primary resonance frequency of the first moving means.
 8. An optical disc apparatus which applies a light spot to an optical disc, thereby to record or reproduce information on or from the optical disc, comprising: first moving means for moving the light spot applied to the optical disc, in a radial direction of the optical disc; an optical head having converging means for converging the light spot on the optical disc; spot position detection means for generating a spot position signal which indicates a positional difference in a radial direction of the optical disc, between a center of the optical head and the light spot on the optical head; tracking error detection means for generating a tracking error signal which indicates a positional dislocation between the light spot and a track on the optical disc; first control means for subjecting the spot position signal or the tracking error signal to a first processing by a phase compensation loop filter, and outputting the signal to the first moving means; second moving means for moving the optical head in a radial direction of the optical disc; second control means for subjecting the spot position signal to a second processing by a traverse loop filter, and outputting the signal to the second moving means; and system operation control means for operating the first control means to perform a phase-lag compensation and a phase-lead compensation by the phase compensation loop filter to the spot position signal, thereafter switching the spot position signal to the tracking error signal to perform the phase-lag compensation and the phase-lead compensation to the tracking error signal, and operating the second control means after operating the first control means. 